Frame aggregation and frame bursting are two proposed mechanisms for enhancing the performance of WLAN systems. Such mechanisms are under consideration for the 802.11n extension to the 802.11 WLAN standard, which will allow for higher throughput WLAN devices. Both TGnSync and WWISE proposals are considering various types of frame aggregation and frame bursting schemes.
FIG. 1 shows different types of frame aggregations and frame bursting schemes that were proposed by either TGnSync, by WWiSE, or by both. Aggregation schemes can generally be differentiated according to which parts of a packet they aggregate.
MSDU aggregation (100) aggregates one or more medium access control (MAC) service data units (MSDUs) 102 to form an aggregated MSDU (A-MSDU) 104, with each MSDU separated by a subframe header 106. A MAC header 108 is added to the A-MSDU to form a single MAC protocol data unit (MPDU) 110.
MPDU aggregation (120) aggregates one or more MPDUs 122 to form a single aggregated MPDU (A-MPDU) 124, with each MPDU separated by an MPDU delimiter 126. A physical (PHY) header 128, including a legacy training and SIGNAL field 130 and an HT Training and SIGNAL field 132, is added to the A-MPDU 124 to form a PHY protocol data unit (PPDU) 134.
PPDU aggregation (140) aggregates one or more PPDUs 142, each PPDU including a PHY header 144 and an MPDU 146. A PHY header 148, including a legacy training and SIGNAL field 150 and an HT Training and SIGNAL field 152, is added to form a single aggregated PPDU (A-PPDU) 154.
PPDU Bursting (160), also known as high-throughput (HTP) Burst Transmission, involves transmitting a sequence of frames 162 by a single high-throughput station (STA) in a single medium access. Each frame 162 includes a PHY header 164, having a legacy training and SIGNAL field 166 and an HT Training and SIGNAL field 168, and an MPDU 170. The frames 162 may be transmitted as part of an A-PPDU, or with reduced interframe spacing (RIFS) 172 to enhance medium efficiency.
Aggregation or bursting schemes can support either aggregating frames destined to a single receiver (i.e., a single WLAN destination), aggregating frames destined to multiple receivers (i.e., multiple WLAN destinations), or both. SRA is used to refer to Single Receiver Aggregation, while MRA is used to refer to Multiple Receiver Aggregation. For example, the MSDU aggregation scheme is typically used for SRA since it contains only one MAC header which can identify a single WLAN receiver address. On the other hand, the MPDU aggregation, PPDU aggregation, and PPDU Bursting schemes can either be used for SRA or MRA, since each MPDU within the aggregate or burst contains a MAC header which can identify a different WLAN receiver address.
Frame aggregation and bursting schemes have the benefit of increasing the efficiency and overall throughput of the WLAN system. A drawback is that most of the aggregation and bursting schemes are not presently supportive/friendly to the issue of saving power/battery. The main problem is that the duration of an aggregated frame or burst can be quite long. So if knowledge about which STAs' data (i.e., which WLAN destination addresses) are contained within an aggregated frame or burst is not provided upfront, then each STA within the WLAN will have to receive and decode the entire aggregated frame or burst in order to check if the frame or burst contains some data destined to the STA.
The act of receiving and decoding the information in such lengthy packets consumes a large amount of energy for the STA's receiver, and significant power/battery savings can be achieved if the receiving STA has some upfront knowledge to indicate that it should not listen to (receive and decode) a particular aggregated frame or burst if it is not an intended receiver.
By providing upfront knowledge about which STAs have data within the aggregated frame or burst, all STAs that do not have data within the aggregated frame or burst can achieve power savings by sleeping (i.e., not listening to or not decoding the full packet) during the duration of the aggregated frame or burst. On the other hand, STAs that do have data within the aggregated frame or burst may be able to achieve power savings if some more upfront information is provided. Such upfront information concerns the timing of the transmission of the STA's data within the aggregated frame or burst. The basic idea is that such STAs will utilize the upfront timing information to wake up (listen and decode) during the portion of the aggregated frame or burst that contains its data, and sleep during the remaining portions that do not contain its data, hence reducing its power consumption.
In prior art, there are some proposals to support power/battery savings. For example as shown in FIG. 2, the A-MPDU aggregation scheme 200 of the TGnSync proposal proposes using an MRAD (Multiple Receiver Aggregate Descriptor) as the first MPDU within an aggregated frame (A-MPDU) that is destined to multiple receivers. The PPDU 202 includes a PHY header 204 and an A-MPDU 206. The PHY header 204 includes a legacy training and SIGNAL field 208 and an HT Training and SIGNAL field 210. The A-MPDU 206 includes an MRAD MPDU 212 and a plurality of MPDUs 214, each separated by an MPDU delimiter 216.
The MRAD MPDU 212 is used in the following fashion. A STA that does not have data within the aggregated frame will receive and decode up until the end of the MRAD MPDU 212, and the STA learns that its receiver address is not included within the MRAD, it can go to sleep (i.e., disable its receiver) until the end of the aggregated frame. Since TGnSync requires that MPDUs destined to the same receiver address have to be placed contiguous to each other within the A-MPDU, a STA that has data within the aggregated frame will receive and decode data until it receives all of its MPDUs and detects a different receiver address in the next MPDU, at which point it can go to sleep (i.e., disable its receiver) until the end of the aggregated frame.
Even though the MRAD mechanism provides a way to achieve power savings in the case of MRA based on A-MPDU aggregation, the MRAD mechanism is appropriate for single-rate MRA aggregation but not sufficiently suited for Multiple-Rate MRA (where the aggregate MPDUs are sent at different rates), for PPDU Aggregation, nor for PPDU Bursting.
One proposal describes MMRA (Multiple-rate (or Multiple MCS) Multiple Receiver Aggregation), which relates to support for power savings that can be achieved when Multiple-rate MRA (MMRA) is used. That proposal includes using an MMRAD (MMRA Descriptor) which contains information on the STA IDs (i.e., receiver addresses) as well as timing offset information, which can be used for power savings. The MMRAD is defined within the MAC portion of the frame, and a single bit within the PHY portion of the frame (specifically within the HT-SIG field) is used to indicate the presence of an MMRAD.
The prior art proposals suffer from many drawbacks, such as: the length of the MRAD or MMRAD is large and inefficient, it is a variable length field, and implementation can be simplified by using only a fixed length packet. Also due to such a large field, the power saving information cannot be embedded within the PHY layer which should be maintained at a small size. Since the power savings information is sent at the MAC level, it is not sufficiently robust because the MRAD is sent at a rate that not all STAs may be able to decode. It is also a MAC MPDU, so if it is lost or if a STA cannot properly decode it, then there are no power savings. Another drawback is that timing information is not provided in an efficient manner. The current proposals mostly apply to A-MPDU aggregation, and cannot efficiently and robustly work with A-PPDU aggregation, PPDU bursting, MRMRA, or reverse direction traffic.